CD-ROM discs are capable of storing large quantities of digital information. This information may be program, database, image, music, video, desktop publishing, or other information susceptible to digital storage. The information is generally stored by physical pits and lands situated along a spiral track that is several miles long. The physical pits may be made optically or by direct physical manipulation, but are readable only optically. That a memory is optically readable means herein that the memory is readable through the use of a laser or other light source.
There are generally two alternate methods that may be used to manufacture CD-ROM's with specific information on them. The first of these is “glass mastering”, which begins with creation of a first copy of the disc which will be used to create a series of positive and negative copies, which will then be used to press identical copies of the first disc. This method, with possibly only a few exceptions, does not allow for the placement of unique information on any one copy. “CD-R” is an alternative writing process, wherein a layer of organic dye allows the CD-R disc to have a unique set of data recorded on it by a CD-R recorder. Copies made by the CD-R method need not be identical. Rather, each may contain certain unique information. Whichever method is used, each copy of the disc is generally coated with a reflective coating and a clear protective layer after being encoded with information.
In FIG. 1, there is shown a sectional side view of a prior art disc, taken along a radius of the disc. The data on the disc is represented by the structure 2 imprinted on the disc. As mentioned, the structure 2 representing the data is generally covered by a clear protective coating 4, after being covered by a thin reflective layer which is not depicted in the figures for simplicity. Generally, the information stored on a CD-ROM is read by directing a low-power laser (typically solid-state or HeNe) at the portion of the spiral track containing the information to be read, along the direction 6. Because the pits reflect light differently than the plains or lands between pits, a photodiode or other photosensitive receiver 8 exposed to the reflected light will detect a series of light and dark reflections as the disc spins, which can be converted to a series of 1's and 0's. These 1's and 0's are a digital representation of the information stored on the disc. Generally, to ensure that the pits and lands pass the laser and receptor 8 at the same rate regardless of location, the disc turns at a lower angular velocity as sectors farther from the center are read.
As a high-density storage medium, CD-ROM technology involves the use of channel codes to retrieve the information stored on the disc. Typically, the channel code used is eight-to-fourteen modulation, a method that relies on transitions to signal ones, and the absence of transitions to signal zeros. Because of this high density and resulting high storage capacity, CD-ROM technology has emerged as a superior alternative to floppy disc technology for marketing large application programs or other memory intensive data. Furthermore, the fact that they are an optical rather than magnetic medium makes CD-ROM's impervious to influences that would corrupt a floppy disc. Even the laser that is typically used by a user machine to read a CD-ROM disc is incapable of damaging or writing on the disc. Hence the moniker, ROM or Read Only Memory.
In the past, the inability of user machines to write on CD-ROM's has caused CD-ROM's to be difficult to secure. Once one user had acquired a disc, there was no way to avoid the dissemination of the information on the disc to other users. In light of this problem with CD-ROM's, certain security measures have developed. In one prior art scheme, a CD-ROM is packaged with a companion floppy disc, which is magnetically readable and writeable by the user computer, to be used along with the CD-ROM to access the information on the CD-ROM. The floppy, which contains information necessary to the use of the CD, is copy-protected. However, the security provided by this scheme is compromised by the widespread availability of means to defeat copy protection on floppy discs.
Methods of copy-protecting a CD itself include the method and apparatus claimed by Kikinis as disclosed in U.S. Pat. No. 5,596,639. The invention of Kikinis involves the use of a high-powered laser to damage the physical structure of the disc, in selected areas, to create a password. Drawbacks to this method are readily apparent; the programming of the password onto the disc requires a higher power laser than that used by the user computer. Thus there is no way, using this method, for the information on the disc to indicate whether the disc has been used already. Consequently, although the disc may be copy-protected, it is not protected from use by more than one user computer.
To overcome the drawbacks evident in the prior art, it is desirable to provide an optically readable memory that prevents unauthorized use. For example, it is desirable to provide a CD-ROM that is capable of changing in response to an authorized use in order to block subsequent unauthorized use. To this end, according to an embodiment, the optically readable memory of the present invention is adapted to be permanently altered by a finite number of read cycles, so that a vanishing code located on the disc, usable to access the disc, becomes unreadable. Alternatively, in an embodiment of the invention, the user machine may alter the optically readable memory by writing rather than obscuring security information.